1. Field of the Invention
The present invention relates generally to perforating tools used in downhole applications, and more particularly to shaped charges for creating an enhanced perforation tunnel in a target formation zone in a well.
2. Background Art
To complete a well, one or more formation zones adjacent a wellbore are perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones. A perforating gun string may be lowered into the well and one or more guns fired to create openings in casing and to extend perforations into the surrounding formation.
With reference to FIG. 1, after a well 11 is drilled, a casing 12 is typically run in the well 11 and cemented to the well 11 in order to maintain well integrity. After the casing 12 has been cemented in the well 11, one or more sections of the casing 12 that are adjacent to the formation zones of interest (e.g., target well zone 13) may be perforated to allow fluid from the formation zones to flow into the well for production to the surface or to allow injection fluids to be applied into the formation zones.
To perforate a casing section, a perforating gun string may be lowered into the well 11 to a desired depth (e.g., at target zone 13), and one or more perforation guns 15 are fired to create openings in the casing and to extend perforations into the surrounding formation 16. Production fluids in the perforated formation can then flow through the perforations and the casing openings into the wellbore.
Typically, perforating guns 15 (which include gun carriers and shaped charges mounted on or in the gun carriers or alternatively include sealed capsule charges) are lowered through tubing or other pipes to the desired well interval on a line 17 (e.g., wireline, e-line, slickline, coiled tubing, and so forth). The charges carried in a perforating gun may be phased to fire in multiple directions around the circumference of the wellbore. Alternatively, the charges may be aligned in a straight line. When fired, the charges create perforating jets that form holes in surrounding casing as well as extend perforations into the surrounding formation.
Various types of perforating guns exist. One type of perforating guns includes capsule charges that are mounted on a strip in various patterns. The capsule charges are protected from the harsh wellbore environment by individual containers or capsules. Another type of perforating guns includes non-capsule shaped charges, which are loaded into a sealed carrier for protection. Such perforating guns are sometimes referred to as hollow carrier guns. The non-capsule shaped charges of such hollow carrier guns may be mounted in a loading tube that is contained inside the carrier, with each shaped charge connected to a detonating cord. When activated, a detonation wave is initiated in the detonating cord to fire the shaped charges. In a hollow-carrier gun, charges shoot through the carrier into the surrounding casing formation.
There have been attempts to optimize the design of shaped charges for producing deeper penetrations into the formation. For example, U.S. Pat. No. 6,152,040 issued to Riley et al. discloses a shaped charge having a liner formed from a metal having a fine, uniform grain structure. The finer grains make it possible to produce less variation in the liner material structure, leading to more symmetric projectile jets to produce deeper perforation tunnels.
U.S. Pat. No. 6,446,558 issued to Peker et al. discloses shaped charges having a liner made of a composite material of fibers or particles of a solid reinforcement dispersed in a solid amorphous matrix. The penetrator jet (projectile) formed from such a liner may operate by two mechanisms: semi-liquid mass and solid mass penetrators, leading to deeper perforation tunnels.
While producing deeper perforation tunnels is desirable, it is equally important that the resultant tunnels are permeable so that the formation fluids can flow into the well. One problem often encountered in perforation operations is that the slug from a molten liner of a shaped charge may be embed in the perforated hole (tunnel), impeding the flow of oil into the well casing. Thus, attempts have been made to improve the design of a liner of a shaped charge such that the damage caused by the liner can be minimized.
A typical liner is prepared from pure metals, alloys, and/or ceramics. U.S. Pat. No. 5,098,487 issued to Brauer et al. discloses copper alloy-based metal liner for shaped charges. Such a liner has a ductile metal matrix and a discrete second phase. The second phase is molten when the liner is accelerated following detonation. The molten phase reduces the tensile strength of the matrix so that the liner slug is pulverized on striking a well casing. The slug does not penetrate the hole perforated in the well casing by the liner jet. As a result, oil flow into the well bore is not impeded.
Published U.S. patent application Ser. No. 2005/0011395 discloses thermal spray techniques for making a liner comprising two reactive components. According to these techniques, reactive components are thermally sprayed together and/or sequentially to build up a “green body” comprising the reactive components. Although a portion of the reactive components may react with each other during the thermal spraying operation, at least a portion of the reactive components remain unreacted in the green body. The reactive components may subsequently be reacted by any suitable initiation techniques, such as from the heat or shock of an explosion.
Published U.S. patent application Ser. No. 2005/0056459 discloses shaped charges having a pressed polymer (e.g., fluorinated polymer) pellet positioned between the explosive charge and the metal liner. The polymer will ignite and burn after being injected into a perforated tunnel. The burning of the polymer helps to stimulate (fracture) the well.
Published U.S. patent application Ser. No. 2005/0115448 discloses shaped charges having liners designed for sandy formation. The liner is low density and has a filler material that is enclosed by a polymer resin skin, such as plastic or polyester. The filler material is in the powdered or granulated form and is left largely unconsolidated. The powdered or granulated materials is a metal powder that is coated with a polymer, thereby permitting a secondary reaction inside the formation following detonation.
While these prior art approaches can produce improved perforation tunnels, there is still a need for improved liners that can produce perforation tunnels with no or minimal damage caused by the liner.